1. Field of the Invention
The present invention relates to an exhaust gas heat exchanger for exchanging heat between the exhaust gas emitted from the internal combustion engine and a cooling fluid, which is effectively applicable to the exhaust gas recirculation (EGR) gas heat exchanger (EGR gas cooler) for cooling the exhaust gas for the EGR system.
2. Description of the Related Art
The EGR gas cooler is used for improving the effect of the EGR, i.e. the effect of reducing the nitrogen oxide in the exhaust gas by cooling the exhaust gas in EGR, and generally used to cool the exhaust gas in EGR utilizing the engine cooling water.
The present inventors have test produced and studied various EGR gas coolers. In all the working models developed, a large amount of particulates of carbon (soot) or the like are deposited on the downstream side of the EGR gas cooler in the exhaust gas flow, and the fins arranged in the exhaust gas passage are clogged, thereby very frequently posing the problem of a reduced cooling performance and an increased pressure loss.
This is by reason of the fact that the exhaust gas generated by combustion contains unburned substances such as particulate matter (soot). Accordingly, as the exhaust gas flows downstream, a decreasing temperature and a decreasing volume of the exhaust gas relatively increases the ratio of particulate matters deposited. As a result, the particulate matters easily attach to the surface of the fins. At the same time, the flow rate of the exhaust gas decreases and thereby it becomes more difficult to blow off the particulate matters attached on the surface of the fins.
The specification of U.S. patent Ser. No. 5,803,162 discloses, as shown in FIG. 30, an invention in which a plurality of sets of two inwardly slanted rectangular louvres 111c are arranged with the ends thereof having a narrower gap directed upstream in the exhaust gas flow. In the cited invention, however, particulate matter is liable to be deposited on the root of the louvres 111c for the reason described below.
Specifically, the exhaust gas that has impinged on the upstream surface of the louvres 111c in the exhaust gas flow rides over the upper end of the louvres 111c toward the downstream surface thereof, and thus flows downstream in the form of a vertical swirl.
However, the height h of the louvres 111c is such that, as shown in FIGS. 31A to 31C, the vertical swirl generated fails to circumvent the louvres 111c to the root thereof. As a result, the exhaust gas flow stagnates at the root of the louvres 111c and cannot blow off the particulate matter attached on the root. Thus, the particulate matter is deposited in an increasing amount on the root.
In view of the facts described above, the object of this invention is to provide a heat exchanger in which the fins are prevented from being clogged.
In order to achieve this object, according to this invention, there is provided a heat exchanger for exchanging heat between the exhaust gas emitted from the internal combustion engine and the cooling fluid, comprising a flat exhaust gas passage (110) allowing the exhaust gas to flow therein and fins (111) arranged in the exhaust gas passage (110) and corrugated, as viewed from the direction of the exhaust gas flow, for promoting the heat exchange between the exhaust gas and the cooling fluid, wherein a plurality of louvres (111c) are arranged on the inner wall of the exhaust gas passage (110) along the exhaust gas flow in such a manner that the distance between the surface (111f) of each of the louvres (111c) and the inner wall of the exhaust gas passage (110) increases progressively downstream in the exhaust gas flow. Further, the surface (111f) of the louvres (111c) forms an angle to the direction of the exhaust gas flow.
As a result, the exhaust gas that has impinged on the downstream side of the upstream surface of the louvres (111c), and rides over the upper end thereof, circumvents the louvres (111c) toward the downstream surface thereof in the EGR gas flow and fails to reach the root of the louvres (111c) as in the invention described in the cited specification. The exhaust gas that has impinged on the upstream side of the upstream surface of the louvres (111c) and rides over the upper end thereof, circumvents the louvres (111c) toward the downstream surface thereof in the EGR gas flow. However, it reaches the root of the louvres (111c) since the height of the louvres (111c) is small.
The exhaust gas that has reached the root of the louvres (111c) is drawn into the exhaust gas stream narrowed and increased in speed by flowing in the gap between the louvres (111c) and the fins (111), and thus flows to the downstream side of the louvres (111c) along the root thereof.
Consequently, the flow rate of the exhaust gas is increased at the root of the louvres (111c), and the particulate matter attached on the root can be blown off, thereby making it possible to prevent the particulate matter from being deposited on the root.
The present invention may be more fully understood from the description of preferred embodiments of the invention, as set forth below, together with the accompanying drawings.